While single-mode fiber is a very efficient, low loss, lightwave transmission medium, and therefore used widely in applications like long haul communications where very low loss is a requirement, multi-transverse-mode optical fiber (hereinafter multimode fiber) fiber has an established status for short haul applications, and specialty fibers. For example, Local Area Network (LAN) applications have traditionally been based on low cost multimode fiber based systems. Such systems enable low cost by utilizing serial transmission, loose alignment tolerances between source and fiber end, and large alignment tolerances within connections and splices. To serve these applications effectively, the industry has set a standard for minimum acceptable transmission distance, and minimum optical fiber performance for that distance. The distance in current use is 300 meters for a single in-building optical link. In-building optical links comprise the vast majority of optical links in use today.
As data rates have increased to 10 Gb/s, the reach of traditional graded index multimode fibers is limited to distances of only 26 to 33 meters, using low cost multimode based serial systems.
The IEEE draft protocol for 10GBASE-LX4 is a four-channel WDM system operating around 1310 nm, that can support up to 300 meters on standard grade multimode fibers. This 10GBASE-LX4 solution has met with only modest success in the market due to high optical complexity and tight single mode tolerances, resulting in high cost.
As a result the 10GBASE-SR (Short Range) Vertical Cavity Surface Emitting Laser-based (VCSEL-based) serial transceivers operating at 850 nm were developed, in parallel with OM3 850 nm optimized multimode fiber. OM3 fibers, per the International Standards Organization (ISO) 11801 designation, are fibers with an effective modal bandwidth (EMB) of 2000 MHz.km at 850 nm. The integration of VCSEL launch and OM3 fiber provides a low cost 300 meter solution for 10 Gbps LAN, or Gbps Storage Area Network (SAN), transmission, and was standardized by IEEE802.3 in 2002. Typical reach is 1.1 km with state of the art OM3 compliant fiber. Since 10GBASE-SR over OM3 fiber is the lowest cost 10 Gb/s 300 meter system, it is the preferred system for new fiber installations.
Yet the industry still lacks a cost effective solution for installed non-OM3 multimode fiber.
Stimulated by the development of dispersion compensation schemes embedded in transmitter or receiver ends (either optical mode filtering or electronic dispersion compensation), IEEE has established the 802.3 10GBASE-LRM (Long Reach Multimode) protocol for 10Gbps single channel transmission at 1300 nm over installed-base Fiber Distributed Data Interface (FDDI) grade, OM2 grade, and OM3 grade, fibers. However, none of the FDDI grade, OM2, and OM3, grade fibers is optimized for 1300 nm performance with laser sources. In simulations by IEEE contributors to date, the 99% 10GBASE-LRM reach is demonstrated to be only 220 m for FDDI grade, but an acceptable 300 m for OM3 fiber using electronic dispersion compensation and an offset launch condition. The cost of 10GBASE-LRM is expected to be higher than 10GBASE-SR, but lower than 10GBASE-LX4 (the four channel multiplexed system), making it potentially viable in the market place for existing multimode fiber installations.
A reasonable next-step in higher speed transmission over multimode fiber would be 40 Gbps. The two most likely candidates are (what would be called) 40GBASE-SX4 supporting 300 meters over OM3 fiber, and 40GBASE-LRM supporting 220 meter over FDDI grade, OM2, and OM3 fibers. The 40GBASE-SX4 would be a CWDM (Coarse Wavelength Division Multiplexing) solution similar to the unsuccessful 10GBASE-LX4 solution, but with some key advantages: 1) Lower cost 850 nm VCSELs would be used instead of DFB (Distributed FeedBack) lasers, 2) Lower packaging cost would be enabled by multimode tolerances 10 times easier than the singlemode tolerances required for LX-4. 3) The use of conventional low cost CMOS integrated circuits is enabled with an SX-4 solution operating at 10 GHz frequencies, vs the more expensive silicon-germanium materials required for the 40 GHz ICs used for serial 40 Gb/s solutions.
The next speed of Ethernet, likely to be captured in IEEE standards, is not 40 GbE but rather 100 GbE, with at least one multimode fiber solution included. One solution would be 10×10 Gb parallel transmission over multimode fiber in ribbons. This solution would be very cost effective up to 100 m, but would become more expensive than CWDM solutions beyond 200 m. A multimode fiber that allows multiple 10 Gb wavelengths on a broadband fiber, with transmitter options that preserve the low cost advantage, may be the most attractive solution for 100 GbE. Such a fiber, however, should maintain the lowest cost value offered today for 1 and 10 GbE transmission at 850 nm.
Thus multimode fibers are needed that are optimized across broad wavelength ranges, coupled with appropriate laser launch conditions, for wavelengths to at least 300 meters, and data rates at 1, 10, and 100 Gb/s. The objective is to design fibers that maintain the present low cost for 1 or 10 Gbps at 850 nm, while opening up other wavelength bands for CWDM of 10 and 20 Gbps transmission.
We have designed a new class of multimode optical fibers optimized for operation across broad wavelength ranges. These are aimed at applications using 1, 10 and 100 Gb/s data rates at 850 nm, as well as lengths of at least 300 meters.
The improved multimode optical fibers have cores doped with aluminum and/or phosphorus. In preferred embodiments they are doped with germanium and phosphorus, or germanium and aluminum. Optical fibers with these multiply doped cores are shown to provide the optical transmission qualities necessary to meet new standards for short haul optical fiber links. More details on these multiply doped optical fibers are set forth in U.S. application Ser. No. 11/511,174, which is incorporated by reference herein.
However, we have also observed that, while these optical fiber designs provide very effective solutions for low cost, high performance, multimode fiber applications, some optical fibers with phosphorus or aluminum doping have unexpectedly high sensitivity to hydrogen contamination. Thus the long term aging qualities of fiber doped with phosphorus and/or aluminum may limit the effectiveness of these optical fibers for some applications.